High performance tires have rubber treads which typically have a relatively high aromatic oil content in order to increase their traction on dry road surfaces.
In particular, while increasing the oil content of a tread rubber composition conventionally increases its dry traction, it also conventionally reduces its durability as evidenced by a reduction of one or more of 300% modulus, breaking strength, Energy to Break and DIN abrasion resistance physical properties.
For example, a tire with a tread rubber composition that is too soft may exhibit good traction but its surface may have insufficient resistance to abrasion and may blister or otherwise physically degrade under long running conditions. On the other hand, a tire tread that is too hard may exhibit good abrasion resistance and associated physical wear characteristics but will likely tend to skid during vehicular cornering. It is believed that such phenomenon is well known to those having skill in such rubber compounding art.
Therefore, a means of providing a high performance tire tread with a combination of both good, dry traction accompanied with good durability, including abrasion resistance, is desired in order to provide a tire with sufficient dry traction so that it can travel at relatively high speeds and sufficient durability so that it can travel over an extended period of time at relatively high speeds.
Accordingly, this invention relates to providing a tire tread rubber composition with a combination of good, dry traction and durability by use of high Tg diene-based elastomers together with a minor amount of a high Tg liquid high vinyl polybutadiene polymer in place of at least a portion of an aromatic rubber processing oil.
In one aspect, while it is believed to be well known to those having skill in such art that while use of higher Tg polymers typically increases hysteresis of a rubber composition and consequently its traction, it is believed that it would normally be expected to reduce its durability, including a resistance to abrasion property, as hereinbefore discussed.
It is similarly believed to be well known to those having skill in such art that while use of higher molecular weight polymers typically improves durability of a rubber composition, it is believed that it would normally be expected to decrease its traction insofar as a tire tread is concerned.
It is a significant aspect of this invention that benefits of the aforesaid desirable abrasion resistance and durability physical properties have been observed to be obtained via the use of a liquid polymer that has both a higher Tg and molecular weight than the rubber processing oil which it, at least partially, replaces in a rubber composition for a tire tread.
In practice, rubber processing oil, particularly aromatic processing oil, is sometimes blended with relatively high viscosity (Mooney viscosity) elastomers in order to reduce their viscosity so that the unvulcanized elastomer may be more easily processed in conventional rubber processing equipment, including internal rubber mixers and various rubber extruders. Such oil extended elastomers may be used, for example, for rubber compositions as components of tires such as tire treads. The use of viscosity-reducing aromatic processing oil in relatively high viscosity elastomers for such purpose is well known to those having skill in such art.
Sometimes, it has been proposed to use various liquid polymers which contain carbon-to-carbon double bond unsaturation to replace at least a portion of such rubber processing oil. The philosophy has been for the liquid polymer to help in reducing the viscosity of the elastomer and to later co-vulcanize with the elastomer upon curing the rubber composition.
Such use of rubber processing oil and proposed use of unsaturated liquid polymers is well known to those having skill in such art.
Indeed, while rubber processing oils have been used to improve the processability of various high viscosity elastomers, the inclusion of processing oils in such rubbers often results in a decrease in its modulus of elasticity. Accordingly, the inclusion of substantial amounts of processing oils in rubber compounds is frequently not always a good option for attaining good processability.
Alternatively, high performance rubber composition requirements, particularly for tire treads, have traditionally necessitated using elastomers of relatively high viscosity and associated very poor processability characteristics. As hereinbefore pointed out, it has been known to add a liquid polymer which is co-vulcanizable with such an elastomer.
The term "phr" as used herein, and according to conventional practice, refers to "parts of a respective material per 100 parts by weight of rubber elastomer". In the description of this invention, the terms "rubber" and "elastomer" can be used interchangeably, unless otherwise distinguished. The terms "rubber composition", "compounded rubber" and "rubber compound" can be used interchangeably to refer to "rubber which has been blended or mixed with various ingredients and materials" and the terms "cure" and "vulcanize" may also be used interchangeably herein, unless otherwise noted and such terms are well known to those having skill in the rubber mixing or rubber compounding art.